In an existing device of this general type, disclosed in U.S. Pat. No. 4,510,847, a proportioning pressure valve acts through a choke on one of the control devices which operates a multiple position control valve. When the choke is open, two other chokes which are similarly connected to this control device do not operate. In this way, the operating mode is set without deactivating first and second chokes. The proportioning pressure valve includes a valve piston which is subject to leakage. This necessitates connecting the proportioning pressure valve to a storage tank.